The present invention relates to automatic temperature controls for washing machines, such as clothes washing and dishwashing machines, and in particular to an automatic temperature control employing a single solid-state switch.
Current high-efficiency washing machines may provide for precise control of water temperature to ensure thorough cleaning while limiting energy consumption. Such systems typically employ a temperature sensor monitoring the temperature of the water received by the wash chamber as a mixed output from two solenoid valves, one receiving water from a hot-water line, and the other receiving water from a cold-water line. By controlling the relative proportion of time that each solenoid valve is on, and monitoring the mixed water, a precise water temperature may be produced. Precise control of water temperature can minimize the amount of hot-water required and hence increase the energy efficiency of the appliance.
Solenoid valves provide only two states: fully on or fully off, but they may be switched between these states or “duty cycle modulated” to control the relative proportion of water passed by each valve and thus to provide an arbitrary temperature of water. Solid-state switching devices, such as triacs, are used to control the high current necessary to switch the valve with low powered signals from a microprocessor. The microprocessor may read in the temperature from a temperature sensor, typically a thermistor, and control the actuation of two output lines leading to two triacs, one associated with each water valve.